To solve the problem that the macro-size, microhardness, and mechanical properties of the laser direct metal deposition (DMD) of thin-walled parts are inconsistent due to the heat accumulation, an improved three-dimensional finite element heat propagation model was developed to simulate the temperature evolution of the single-pass multi-layer thin-walled parts. The results show that the heat accumulation is contributed to the transformation of the heat dissipation mechanism from three-dimensional to two-dimensional, while the residual heat of the former layer has no significant effect on the heat accumulation of the current layer. The optimized strategy of decreasing laser power gradually is more effective than the optimized strategy of increasing the time interval between layers in maintaining the stability of the molten pool. The integrated optimized strategy of parabolic-shape laser power decreasing and time interval of 5 s is the most effective method to reduce the heat accumulation in the DMD of thin-walled parts. The thin-walled parts with an optimized deposition process have a more uniform width, finer microstructure, and higher microhardness. This work provides theoretical guidance for reducing the heat accumulation during DMD thin-walled parts and is beneficial to improving the uniformity of mechanical properties of thin-walled parts.